Torque sensors that detect torque acting around a predetermined axis of rotation are widely used in various transportation machines and industrial machines. For example, the following Patent Literature 1 discloses a torque sensor of a type where mechanical deformation generated by action of torque is detected by a strain gauge. Patent Literature 2 discloses a sensor formed with a magnetostrictive film by plating processing on a surface of a shaft thereof. The sensor detects torque acting on the shaft by measuring a change in magnetic characteristics of the magnetostrictive film. Meanwhile, Patent Literature 3 discloses a torque sensor of a type where a magnetism generation unit is provided at an end portion of a torsion bar and a variation in a magnetic flux density of magnetism generated by the magnetism generation unit is detected using a magnetism collecting ring. Patent Literature 4 discloses a torque sensor of a type where a number of magnets are arranged in a, cylindrical shape such that N poles and S poles are alternately aligned in a peripheral direction thereof and a magnetic field generated by these magnets is detected. Patent Literature 5 further discloses a torque sensor that includes a link mechanism to allow the shape of a ring-shaped member to be deformed in a radial direction by action of torque and detects, by a load sensor, force applied in the radial direction by deformation of the ring-shaped member. Cited Literature 6 discloses a torque sensor of a capacitance type that detects torque based on a variation amount in a capacitance value of a capacitive element attributable to deformation occurring in a ring-shaped elastic ring by action of torque.
Such torque sensors have been increasingly used in wider applications to life support robots in recent years and thus high safety is desired. However, for example the torque sensor of the current capacitance type includes a mechanical unit, a detection unit of capacitance, and an electronic circuit including a microcomputer but may disadvantageously fail due to condensation, an impact, an overload, or contamination between a pair of parallel flat plates providing the capacitance.
As a simple method for determining whether a torque sensor is out of order, disclosed is a method to align the multiple (e.g. three) torque sensors described in Cited Literature 6 in parallel along an axis of rotation of a torque to be detected and to evaluate differences of output signals of the respective torque sensors. In this method, each two of the three output signals are compared and the torque sensor is determined as normally functioning when the difference between the output signals of the two torque sensors is within a predetermined range. On the other hand, the torque sensor is determined as not normally functioning (out of order) when the difference is not within the predetermined range.